Demons, unlike human cultivators, kept the beastial nature they were born with, so there was no such thing as a mental demon for them. But if one ate the meat of someone who died in vain, there'd be bloody grievous energy on him, which prevented them from entering the immortal realm. The Lightning Tribulation of ascension would cleanse all the grievous energy from a demon beast. Therefore, however high the cultivation of a demon beast, so long as it ate people, eight or nine out of ten would die being hit by lightning. This was also why there were few demonic immortals.